1. Technical Field
The present disclosure relates to a toner, a developer, and an image forming apparatus.
2. Description of Related Art
In an electrophotographic image forming apparatus or electrostatic recording device, an electric or magnetic latent image is developed into a toner image. For example, in electrophotography, an electrostatic latent image is formed on a photoreceptor and is developed into a toner image. The toner image is transferred onto a recording medium, such as paper, and fixed thereon by application of heat, etc.
Toner generally comprises resin particles in which colorant, charge controlling agent, etc., are dispersed, and is manufactured by various processes, such as pulverization, suspension polymerization, dissolution suspension, emulsion aggregation, phase-transfer emulsification, and elongation polymerization.
The resin particles may comprise, for example, a thermoplastic resin such as styrene-acrylic resin, polyester resin, and polyol resin. Polyester resin has superior strength and stability as well as a lower softening point while having a greater molecular weight and a higher glass transition temperature compared to styrene-acrylic resin. Therefore, polyester resin is widely used for toner especially requiring low-temperature fixability. In particular, polyester resin is widely used for toner for full-color printing.
Binder resin generally occupies 70% or more of toner composition. Most binder resins are derived from petroleum resources now being exposed to depletion. Petroleum resources cause a problem of global warming because they discharge carbon dioxide into the air when consumed. On the other hand, binder resins derived from plant resources have been proposed and used for toners. Because plant resources have incorporated carbon dioxide from the air in the process of growing, carbon dioxide discharged from plant resources is merely circulated between the air and plant resources. Thus, plant resources have the potential to solve the problems of both depletion and global warming.
Japanese Patent No. 2909873 (corresponding to Japanese Patent Application Publication No. H07-120975) describes a toner including a polylactic acid as a binder resin. Polylactic acids, derived from plant resources, are widely used and easily available. Japanese Patent Nos. 3347406 (corresponding to Japanese Patent Application Publication No. H07-33861) and Japanese Patent Application Publication No. 59-096123 describe that polylactic acid is obtainable by dehydration condensation of lactic acid monomer or ring-opening polymerization of cyclic lactide of lactic acid. Polylactic acid generally includes a larger content of ester groups than polyester resin. Ester group consists of carbon atoms only. It may be difficult to adjust toner properties with polylactic acids only.
Attempts to use polylactic acid in combination with another resin or to copolymerize polylactic acid with another resin have been made. Japanese Patent No. 3785011 (corresponding to Japanese Patent Application Publication No. 2001-166537) describes a toner including a biodegradable polylactic acid-based biodegradable resin in combination with a terpene phenol copolymer. Polylactic acids are poorly compatible with or dispersible in polyester resins or styrene-acrylic copolymers that are widely used as binder resins. This may be disadvantageous in terms of controllability of toner surface composition that has an influence on toner properties such as storage stability, chargeability, and fluidity.
It is generally difficult for toner to achieve low-temperature fixability and heat-resistant storage stability at the same time. Japanese Patent Application Publication No. 2004-310018 describes a toner including a low-molecular-weight polyester resin in combination with a high-molecular-weight polyester resin obtained by elongating a prepolymer. The toner is designed so that the low-molecular-weight polyester resin contributes to low-temperature fixability and the high-molecular-weight polyester resin contributes to hot offset resistance and heat-resistant storage stability. However, the high-molecular-weight polyester resin may inhibit fixation of the toner on paper. Mixing low-molecular-weight and high-molecular-weight resins is insufficient to obtain a toner having both low-temperature fixability and heat-resistant storage stability.
Lowering thermal properties of resin improves low-temperature fixability but degrades heat-resistant storage stability and hardness of the resin. A resin with a low hardness may cause various problems such as toner filming and deterioration of chargeability.
Japanese Patent Application Publication No. 2008-262179 describes a toner including a block copolymer resin of a polyester having a polylactic acid backbone having a specific D/L ratio with another polyester, in combination with another resin. In an electrophotographic image forming apparatus, more than half of the electric power is consumed in a fixing device. To more save energy, this toner is designed to be fixable at low temperatures.